Digital circuits are often employed with one or more clock signals. At high frequencies, however, these digital circuits may radiate signals as electromagnetic energy that may interfere with the operation of surrounding equipment. Since these emissions are based upon clock signals, energy “spikes” often occur at these clock signal frequencies and their harmonic frequencies. Shielding techniques are often employed to reduce these emissions within certain frequency ranges.
In addition, spread spectrum techniques are often employed to spread the emitted energy over a wider frequency range, thereby decreasing the energy at any given frequency. One technique varies the clock frequency over a range of frequencies such that the average frequency is the desired clock frequency, but the emitted energy is now “spread” over the range of frequencies. Such spread spectrum techniques reduce the interference from high energy spikes at the clock frequency.
Clock signals are often generated using a phase-locked loop (PLL) circuit. A PLL circuit generates a periodic output signal that has a constant phase and frequency with respect to a periodic input signal. In a charge-pump PLL, for example, as described in Floyd M. Gardner, “Charge-Pump Phase-Lock Loops,” IEEE Trans. Communications, vol. COM-28, 1849-1858 (November 1980), a phase detector compares the phase of an input reference clock signal to the phase of a feedback signal derived from the PLL output. The phase detector generates an UP or DOWN error signal indicating the phase difference.
A charge pump generates a charge based on the error signal, where the sign of the charge indicates the direction of UP or DOWN. The charge is either added to or subtracted from the capacitance in a loop filter, based on whether the error signal was an UP signal or a DOWN signal. The loop filter operates as an integrator that accumulates the net charge from the charge pump. The resulting loop-filter voltage is applied to a voltage-controlled oscillator (VCO). The VCO generates a periodic output signal having a frequency that is a function of the VCO input voltage. Input and feedback dividers may optionally be placed in the input and feedback paths, respectively, if the frequency of the output signal is to be either a fraction or a multiple of the frequency of the input signal.
In one exemplary spread spectrum technique, a clock frequency is varied by modifying the feedback divider used to control the output clock frequency of the PLL. The feedback divider typically divides the output signal of the VCO by a fixed number N to generate a signal close in frequency to the input reference clock signal. By varying the value of N, the divided output of the VCO applied to the phase detector also varies the output frequency of the VCO. Spread spectrum techniques of the prior art typically vary the frequency in discrete steps by reading successive values for N from a table stored in memory and supplying the successive values of N to the feedback divider.
U.S. patent application Ser. No. 10/644,362, entitled “Spectrum Profile Control for a PLL and the Like,” incorporated by reference herein, discloses a spread spectrum technique where the spreading of the frequency spectrum of a timing recovery circuit, such as a PLL, is controlled by periodically calculating each value for a divisor, N, of a fractional divider in the feedback path of the PLL. The fractional divider divides the output signal of a VCO of the PLL by the divisor, N, and the value for the divisor, N, is periodically updated based on a spreading profile. The output of the fractional divider and a reference clock signal are provided to a phase detector of the PLL so as to cause the PLL to slew the output frequency of the PLL in accordance with the spreading profile.
While such conventional spread spectrum techniques generate a frequency with a predefined offset from a reference frequency, they suffer from a number of limitations, which if overcome, could further improve the efficiency and utility of spread spectrum techniques. In particular, with such conventional spread spectrum techniques, the VCO output can be used only for the frequency offset or spread spectrum destination. The VCO output cannot be shared with other circuits requiring a constant VCO frequency. A need exists for methods and apparatus for generating a frequency with a predefined offset from a reference frequency that provide predominantly digital spread spectrum or rate offset frequency generation.